THERE'S a lot of concern about climate change resulting from the buildup of carbon dioxide and other ``greenhouse'' gases in the atmosphere.
But we're forgetting about the plants. It now is clear that changes in plant cover - whether they are man-made or in response to climate change - can themselves have a big effect on climate.
Jonathan A. Foley and several colleagues at the University of Wisconsin at Madison make this point with computer simulations backed up by data on actual climate changes in the relatively recent past.
As Professor Foley puts it, their work ``demonstrates that the vegetation cover of the Earth is an important part of the climate system.''
The influence of plants can be dramatic. Warm the Northern Hemisphere climate a bit, and northern (boreal) forests will expand farther north. The new (dark) forest cover will absorb more solar energy than the bare tundra the forest replaces. Also, there is less ice and snow. Such changes can nearly double the effect of the original warming.
Neither scientists nor politicians trying to anticipate climate change can afford to neglect this part of the climate system any more. Just looking at the possible effects of atmospheric changes will mislead them.
The notion that changes in plant cover can affect climate isn't new. Many computer simulations have studied what might happen if tropical forests disappeared through excessive logging, for example.
However, scientists have paid relatively little attention to the role of plants as an integral part of the climatic system. In other words, instead of regarding plant cover changes as an external influence on climate or merely a result of climate change, scientists have to deal with them as changes to the climate system itself.
Gordon Bonan, David Pollard, and Starley Thompson at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., pointed this out two years ago in a research paper in the journal Nature. They ran simulations of Northern Hemisphere climate change on NCAR's supercomputer. They reported that these simulations suggested that ``future redistribution of boreal forest and tundra vegetation ... could initiate important climate feedbacks, which could also extend to lower latitudes.''
Foley, working with John Kutzbach, Michael Coe, and Samuel Levis in Wisconsin, realized that this could explain why the climatic record conflicts with some computer simulations of past climate.
Changes in Earth's orbit 5,000 to 8,000 years ago warmed the Northern Hemisphere slightly. The simulations had underestimated the high-latitude warming by almost 50 percent. When the Wisconsin team threw in forest expansion, their new simulation got it right.
The orbital changes gave Earth's axis a little more tilt in relation to the sun than it had as part of a continuing cycle of change. They also brought our planet closest to the sun in the Northern Hemisphere summer rather than in winter, as is the case today. As the Wisconsin scientists reported earlier this month in Nature, the orbital changes would warm the high northern latitudes by two degrees C or more in summer, fall, and winter. However, the advance of the forest northward over what had been tundra adds something like 4 degrees C additional warming in spring and about 1 degree C in other seasons.
The point here is not merely that plant cover changes boosted regional warming. What is significant is the interplay between those changes and the climate change induced in the first instance by an external cause.
It shows that the problem of anticipating any man-made climate change involves far more than carbon-dioxide warming. Scientists have to take account of a complexity of interacting factors which they are only beginning to understand if they are to explain how the climate changes as it has always been changing ever since the Earth was formed.